In oil production processes, particularly off-shore oil production processes, a mixture of oil and water is often recovered. The water recovered in this way is unwanted and needs to be disposed of. However, it is not environmentally appropriate to dispose of the water while it is still contaminated with oil. Therefore, there is a need to separate the oil from the water.
In practice, there are certain limitations on the apparatus that can be used for this separation. In particular, the constraints of offshore oil production, such as the size of the offshore platform, require a separation apparatus that is both effective and relatively compact. Over recent decades, cyclone separators have been developed to carry out this task.
The operation of a cyclone separator can be described as follows. A fluid (in this case, a mixture of oil and water) enters a cyclone tangentially, causing the fluid inside the cyclone to spin. This creates a radial force that directs the heavier phase (in this case, the water) towards the edges of the cyclone, thus retaining the lighter phase (in this case, the oil) in the centre of the cyclone. The two phases of oil and water can then be extracted from the cyclone separately. The water is extracted via a clean water outlet while the oil is extracted via a waste water reject line.
Compared with alternatives such as settling or skim tanks, a cyclone separator yields a much faster separation within a smaller space. This is because the gravitational force at work in settling or skim tanks is replaced by radial forces in the cyclone of a far higher magnitude. These high forces mean that cyclones are insensitive to motion and orientation, making them particularly ideal for offshore applications in the oil industry.
Nevertheless, there remain difficulties in implementing effective cyclone separators at reasonable cost with required reliability. For example, it is necessary to apply a back pressure to the cyclone from the clean water outlet in order to ensure that the oily waste product is forced through the reject line. This is achieved using a control valve which is placed on the water outlet across which a pressure differential is established and which dissipates pressure energy through turbulent friction.
The implementation of a control valve creates a number of complications in the effective performance of the cyclone. For example, it is often appropriate to have further separation processes downstream of the cyclone to separate out any residual oil in the clean water outlet. The efficiency of these downstream processes typically depends strongly on the size of the oil droplets within the mixture. However, the action of the control valve introduces turbulence into the outlet which causes shear forces that tend to reduce the average oil droplet size.
Another complication arises when trying to control the overall flow of liquid through the system. For example, the control valve may be adjusted to change the flow of liquid to processes upstream and/or downstream of the cyclone. While it may be relatively simple to use an adjustable valve for this purpose, complications arise in that it is typically desired to extract a constant ratio of liquid through the water outlet and the reject line. In particular, it is undesirable to remove too high a proportion of liquid through the water outlet, since this is liable to cause contamination, while it is also undesirable to remove too small a proportion of liquid through this route since this would increase the amount of water flowing through the reject line.
In order to maintain a constant volumetric ratio of liquid through the water outlet and the reject line, it has been proposed to place an additional outlet valve on the reject line. Moreover, a control mechanism is introduced so that the control of the control valve on the water outlet and the additional control valve on the reject line are linked so as to ensure the necessary constant ratio. However, these additional parts carry an expense and risk of failure which is undesirable.
There is an ongoing desire to improve separation apparatuses for use in offshore oil operations and elsewhere. In particular, there is a desire to increase the efficiency of separation while simultaneously retaining reliability and avoiding prohibitive expense.